It is known in the art to couple a vibration shaker to a slip plate supported on a support block, typically of polished granite on which there is a thin oil film maintained, and to oscillate the slip plate in one or another predetermined direction by operation of the shaker. The direction of vibratory movement is critically maintained for test validity. The slip plates are limited in their freedom of movement to have a definite response to shaker driving, e.g. by the use of rails, channels, or other guiding devices such as linkages. Such expedients do not permit driving of the slip plate in two axes simultaneously, or only with the need simultaneously to drive through angles and with added mass.